Normally mass spectrometers are able to separate charged particles in particular ions of atoms or molecules according to their mass-to-charge ratio m/z. That means that ions having the same mass-to-charge ratio m/z have same trajectories in at least parts of the mass spectrometers. For the simplification of the presentation in the following description and patent claims instead of the mass-to-charge ratio m/z only the term mass m will be used. So the term mass m will replace the correct term mass-to-charge ratio m/z. So the reader should take always into account that whenever the term mass m is used it is meant the mass to ratio-to-charge m/z. So for example a function f(m) is not a function of the mass m, it is a function of the mass ratio m/z (function f(m/z)). For example single charged ions 16O+ and double charged ions 32S++ have the same nominal mass-to-charge ratio 16. This means if in the further description a ion having mass 16 is mentioned both ions are described.
At the present the quadrupole mass analyzers of mass spectrometer are calibrated on its own to calibrate the RF voltage and a DC voltage which are applied to the electrodes of the quadrupole just if they are part a mass spectrometer having more than one mass analyzer like a triple quadrupole mass spectrometer comprising three quadrupoles. A quadrupole is operable as a pre-selecting mass analyzer in a mass selecting mode selecting masses in a mass filter window having a filter window width w. For this mode for the amplitude of the applied RF voltage a first function RF(m, w) of a selected mass m and the filter window width w and the applied DC voltage a second function DC(m, w) of the selected mass m and the filter window width w has to be defined by a calibration process. Typically during the calibration of a mass analyzer the analyzer is scanning several calibration masses in one run and afterwards certain parameters of the first function RF(m, w) and the second function DC(m, w) are adjusted by a fitting process. Very often for this fitting there is assumed for the functions RF(m, w) and DC(m, w) a specific function whose flexible parameters can be only changed by fitting the whole scan result to the specific function. Hereby the fitting is very inflexible because functions deviating from the assumed specific function are not possible and excluded be better calibration functions.
By this kind of calibration mass scans over the whole mass range of the calibration masses have to be repeated as long as the calibration functions RF(m, w) and DC(m, w) do not fulfill the required quality conditions.
Mostly these calibration processes are only successful—particularly after a few runs and therefore a short time—if good priority assumptions for the calibrations functions can be made at the beginning of the calibration. Depending on the technical detail of a quadrupole this is not possible for any construction of a quadrupole.
Further on during a mass scan of the calibration process over a mass range there can be detected outliers in the scan resulting from technical instabilities which are not always avoidable. These outliers lead to failure in the calibration process and the calibration results.
It is the object of the invention to improve the calibration of mass spectrometers having at least two mass analyzers. The improved method for calibrating a mass spectrometer shall be faster than the methods of the state of art. Further on the improved method for calibrating a mass spectrometer shall be more robust because it shall be e.g. more independent on the choice of start conditions of the calibration. Further on it is object to define a method for calibrating a mass spectrometer which is flexible. This means that the method may e.g. not relate on start conditions and is able to run with various fitting algorithms and fitting functions to find calibration curves. Another object of the invention was to find a method of calibration which is able to use calibration masses for the calibration having overlapping signals in the operation mode of the mass analyzer which shall be calibrated.